Oversheeting 'Big 6' fibre cement roof

[Glenn]

Hello, I have some reasonably large sheds whose fibre cement roof has definitely reached end of life. I am a big corrugated iron fan, so that's be my default replacement option. Someone has suggested to me why not oversheet rather than go through the pain of (likely Type C asbestos) sheet removal and disposal. Leaving it in place would also help with the condensation challenge. my budget is also insufficient for eg insulated metal panels

 

Research on oversheeting brings up a specialist GRP product with the same Big 6 profile so it sits snugly over. Or with say box profile sheets there are multiple recommendations to first batten out the roof and then lay the metal sheets on the battens. I guess it makes sense - provide a flat plane for the sheets to be screwed into. But it does leave eaves gaps to be dealt with

 

My dumb questions are

- what does the battening achieve that simply laying the box profile sheets onto the fibre cement sheets and drill and screw through both to the wooden purlins below? I have to do that for the battens anyway

- I don't have the budget for any insulation, that can come later, but any benefit from a vapour barrier sandwich between the existing roof and the new sheeting?

 

Regards

Glenn

[Iceverge]

Yikes......

 

Sounds like a recipe for a forever roof.Not in a good way unfortunately.

 

What you have at the moment can be tackled systematically and safely. 

 

If you layer up new roof on old asbestos noone will want to come within a mile of it .

 

If you take appropriate precautions you can remove the Asbestos yourself and get it disposed correctly. If you're able to take the time and care it'll save a lot vs a contractor. 

 

[markc]

The Big6 profile metal sheets “should” lay over the top, but won’t! The original sheets are unlikely to be dead straight and the fixings stick up etc. as above, remove the existing and replace. Over batten and sheet is pretty straight forward but the old sheets are likely to crack in multiple locations and you will regret not doing it right once.

[Iceverge]

Regarding insulating insulated sheets aren't tremendously expensive. 

 

Even a minimal 25mm or so of Insulation would be worth it. 

 

If you really can't afford it or just want to stop condensation drips I would layer a run of chicken mesh over the purlines to support roofing felt and then use standard sheeting. 

 

There's a roof very almost level done on a stable here and it has never dipped.